“We are reacting to a report of a real-world active shooter,” a spokesperson for the base told WTTG TV. “The call occurred during an exercise. Because the report came in as a real active shooter, we have to respond accordingly. We are treating as if it is and no further information at this time.”

Homeland Security Secretary Jeh Johnson referred to an “unfolding situation” at Andrews in his Senate hearing.

The most recent active shooter situation on an airbase was in April, when two Air Force members were killed in a murder-suicide at Lackland AFB in Texas.